1. Field of the Invention
The invention relates generally to the synthesis of zeolites. More particularly, it relates to the synthesis of certain zeolites having large-crystal structures.
2. Discussion of the Prior Art
Zeolitic materials, both natural and synthetic, have been known in the past to have catalytic capability for various types of hydrocarbon conversion reactions. Certain of these zeolitic materials comprising ordered porous crystalline aluminosilicates have a definite crystalline structure, as determined by X-ray diffraction, within which there are a number of small cavities which are interconnected by a number of still smaller channels. These cavities and channels are precisely uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption purposes molecules of certain dimensions while rejecting those of larger dimension, these materials have commonly been known to be "molecular sieves" and are utilized in a variety of ways to take advantage of the adsorptive properties of these compositions.
Crystalline aluminosilicates have been characterized by the presence of aluminum and silicon, the total of such atoms to oxygen being 1:2. The amount of alumina present in conventional aluminosilicates appears directly related to acidity characteristics of the resulting product. Low alumina content is advantageous in attaining low acidity, desirable for low coking and low aging rates.
U.S. Pat. No. 3,941,871 discloses the preparation of a ZSM-5 type zeolite having a high SiO.sub.2 to Al.sub.2 O.sub.3 ratio. The disclosure of this patent includes a teaching of the combined use of TPA and TMA to make a crystalline metal organosilicate. U.S. Pat. No. 3,849,463 discloses a method of decreasing silica occlusion in a zeolite having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of greater than 6 by incorporating in the reaction mixture an alkali metal salt.